Skeletal muscle will be from limbs that are rehabilitating from atrophy caused by a prior 7-day period of hindlimb immobilization in rats. The specific aims of the research plan are to: (1) Determine the synthesis rates of actin and cytochrome c in skeletal muscles at the 6th hr, 2nd day and 4th day of recovery from the prior 7-day immobilization. Synthesis rates are estimated in vivo by the constant infusion of 3H-tyrosine, measurement of the specific radioactivity of either purified actin or purified cytochrome c, measurement of the specific radioactivity of tyrosyl-tRNA and calculation of the estimated rate of fractional synthesis per day; (2) Determine the role of anabolic steroids and of various exercises (treadmill running, weight lifting and electrical stimulation) in addition to "normal" cage activity on the synthesis rates of actin and cytochrome c in skeletal muscles on the 2nd day of recovery from the prior 7-day period of hindlimb immobilization; (3) Determine the threshold of exercice duration needed to increase the synthesis rate of actin and cytochrome c in skeletal muscle on the 2nd day of recovery from a prior 7-day period of hindlimb immobilization; (4) Determine the sensitivity of increasing exercise duration on the amount of increase in actin or cytochrome c synthesis rates; (5) Determine the content of Alphaactin mRNA and of cytochrome c mRNA in skeletal muscles at the 6th hr, 2nd day, and 4th day of recovery from a prior 7-day period of hindlimb immobilization. The content of Alphaactin mRNA or cytochrome c mRNA will be semiquantitated by dot hybridization with cDNA for Alphaactin mRNA from rat skeletal muscle and a genomic clone of rat liver cytochrome c, respectively; and (6) Determine the content of Alphaactin mRNA and cytochrome c mRNA in skeletal muscles in the 3rd and 24th hours after an exercise bout when exercise is preformed on the 2nd day of recovery from a prior 7-day period of limb immobilization.